The present invention relates to a process for preparing a purified terephthalic acid. More particularly, it relates to an improved process for preparing a purified terephthalic acid from a crude terephthalic acid obtained by a liquid phase oxidation of para-dialkylbenzene. In terephthalic acid obtained by a liquid phase air oxidation of paraalkylbenzene such as paraxylene, relatively large amounts of impurities such as 4-carboxy-benzaldehyde(hereinafter referring to as 4-CBA) are included. These impurities adversely affect such polymerization reactions as the preparation of polyethyleneterephthalate which are present in the crude terephthalic acid which is used as the starting material. The impurities also cause the undesired coloring of the resulting polyethyleneterephthalate.
Accordingly, a need has existed for a method by which the impurities such as 4-CAB can be eliminated from the terephthalic acid. Various methods have been proposed for effecting the separation of 4-CBA from crude terephthalic acid. Among them, the method of purifying a crude terephthalic acid by contacting hydrogen with an aqueous solution of crude terephthalic acid in the presence of a catalyst to reduce 4-CBA to para-toluic acid or para-methylolbenzoic acid, which has certain advantages and is disclosed in USP al No. 3,584,039. The separation of para-toluic acid is easier than the separation of 4-CBA, however the former method also requirs special careful operation.
In another method of purifying terephthalic acid by separating impurities such as 4-CBA without the above-mentioned disadvantages, crude terephthalic acid which is obtained by the oxidation of p-dialkylbenzene is dissolved in water at high temperature under high pressure and then the aqueous solution of crude terephthalic acid is contacted with a catalyst of palladium on active carbon to yield a terephthalic acid product of high purity (hereinafter referred to as Pd/C catalyst) (Japanese Patent Publication No. 33189/1974).
According to said process, most of 4-CBA is converted to benzoic acid by a decarbonylation in the presence of the Pd/C catalyst, and the resulting benzoic acid has remarkably higher solubility in water than that of terephthalic acid and is easily removed by the crystallization. However, when the Pd/C catalyst is used for a long time, the catalytic activity is decreased to cause an imcomplete decomposition of the impurities such as 4-CBA and to remain the impurities in the resulting terephthalic acid. In order to prolong the life of the Pd/C catalyst, it is considered to use a crude terephthalic acid having higher purity or to increase an amount of the Pd/C catalyst.
However, these considerations are disadvantageous in industrial operations. Through further studies it has now been found that in the process a contacting crude terephthalic acid with the Pd/C catalyst, carbon monoxide which is formed by the reaction, is dissolved in the liquid phase and the catalyst is poisoned by the dissolved carbon monoxide which results in a decrease in catalyst activity for the decomposition of 4-CBA and which shortens the life of the catalyst. It has also been found that the carbon monoxide, which is dissolved in the liquid phase, can be removed from the liquid phase to the gaseous phase by decreasing the pressure in the catalyst contacting step thereby prolonging the lifetime of the catalyst.